What is called diesel combustion, in which fuel is directly injected into compressed air in the combustion chamber, self-ignites, and is burned by diffusion combustion, is advantageous over spark-ignition combustion in its excellent thermal efficiency. Although fuel generally used in diesel combustion is light oil having a relatively low self-ignition temperature, PTL 1, for example, discloses a technology in which natural gas having a relatively high self-ignition temperature is used as fuel in diesel combustion. Specifically, fuel injection is performed in a predetermined region in the combustion chamber in an early or middle stage of the compression stroke, and the air-fuel mixture formed in the aforementioned region is ignited at a time just before the top dead center of the compression stroke, to generate a high-temperature, high-pressure condition enabling self-ignition of natural gas in the combustion chamber. In addition, fuel is injected into the combustion chamber in a high-temperature, high-pressure condition for diffusion combustion after the top dead center of the compression stroke.
Attempts for igniting gasoline by self-ignition and burning it by diffusion combustion in spark-ignition gasoline engines have also been made. For example, in the technology disclosed in PTL 2, a first fuel injection is performed in a period in the first half of the compression stroke to form substantially homogeneous air-fuel ratio in the entirety of the combustion chamber, and the air-fuel mixture formed by this fuel injection is spark-ignited. Thereafter, a second fuel injection is performed to burn the fuel thus injected, so that the remaining fuel self-ignites with a rise in the temperature and pressure in the combustion chamber resulting from the burning. This technology can enlarge the operation range in which the occurrence of knocking can be prevented, whereby the advantage of diesel combustion can be enjoyed.
PTL 3 discloses technology pertaining to diesel combustion in a gasoline engine for the purpose of knocking suppression, as with the technology disclosed in PTL 2. Specifically, in a relatively high load operation range of a gasoline engine in which knocking is likely to occur, a first fuel injection is performed in a period before the top dead center of the compression stroke, and the fuel thus injected is burned by spark ignition, and fuel injection for diffusion combustion is performed in a period after the top dead center of the compression stroke in which the pressure in the combustion chamber is high due to the burning. In this technology, the timing of the first fuel injection is adjusted in accordance with the engine speed to achieve combustion favorable for suppression of knocking.